Organic Hot Electron Photodetectors Based On Metal/tris-(8-hydroxyquinoline)Aluminum Schottky Junction

With the development of image sensor and communication industry,the demand for flexible,low-cost and high response speed photodetectors(PDs)is growing.In many types of PDs,junction PDs has been systematically studied,because the built-in electric field of junction PDs can effectively dissociate excitons and accelerate the collection rate of carriers by electrodes,so it has been systematically studied.However,the traditional junction PDs are limited by the absorption band gap of semiconductors.Using the hot carrier emission effect in the metals/semiconductors Schottky junction,the PDs with photon energy less than the band gap can be effectively realized.In order to improve the hot carrier emission,micro nano structures with surface plasmon are usually introduced.At present,the hot carrier PDs are mainly based on inorganic semiconductors.The research of organic hot carrier devices is relatively scarce.The main works are as follows:(1)To compared with inorganic semiconductors,organic semiconductors have many merits such as flexibility,variety,adjustable band gap and low price.Therefore,in this paper,we propose to use an organic small molecule luminescent material tris-(8-hydroxyquinoline)aluminum(Alq₃),as organic semiconductor layer.The alloy cathode is composed of high work function Agnano particles(NPs)and aluminum film.An organic hot electron PD with ITO/Alq₃/AgNPs/Al structure was fabricated on ITO glass substrate.Two back-to-back Schottky junctions are formed between the cathode and anode of the device,which effectively suppress the dark current.Fine photoelectric detection achieved at 660?850 nm utilizing hot carriers produced by metal nanostructures,where organic semiconductor Alq₃ is inability to absorb light.At the same time,in the absorption wave range of Alq₃ at 375 nm,self powered detection can be realized under 0 V bias.And in the atmosphere due to the formation of the oxide layer of the composite electrode,the device has strong air stability.(2)The analysis of the device shows that the transport of hot electron excited by AgNPs in semiconductors is mainly completed by drift motion due to its low concentration;the transport mode of free carriers generated by semiconductor absorption is mainly dependent on diffusion due to its high concentration,and the loss of free carriers in the area without internal electric field is small.Applying different bias voltage will lead to the change of the built-in electric field width of the two Schottky junctions,which makes the hot electron PDs show different log I-V characteristics in the absorption and non absorption range of the semiconductor layer.(3)In the experiment,we found that the metal silver with strong activity can induce bad physical and chemical reaction at the Schottky interface,reduce the quality of Schottky junction,and reduce the repeatability of organic hot electron PDs.For this reason,we also propose an organic hot electron PDs with ultrathin BCP insertion layer at the Schottky junction interface of composite electrode/Alq₃,which greatly improves the repeatability from 30%to 90%and makes the photodetector a big step forward in practical application.Furthermore,the work function of the composite electrode is further improved,making the dark current keep lowering.